Add dynamic text contrast for mood colors and make theme preview full screen

- Add WCAG-compliant luminance calculation to Color extension
- Add contrastingTextColor method to MoodTints for automatic black/white text
- Update 12+ entry styles to use dynamic text colors instead of hardcoded white
- Change theme preview sheet to full screen presentation

Text now automatically switches between black and white based on background
brightness, fixing readability issues on light mood colors like yellow (Good).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Shared/Views/CustomizeView/SubViews/AppThemePickerView.swift

@@ -252,8 +252,7 @@ struct AppThemePreviewSheet: View {
                 }
             }
         }
-        .presentationDetents([.medium, .large])
-        .presentationDragIndicator(.visible)
+        .presentationDetents([.large])
     }
 
     private var heroSection: some View {